Measuring hip muscle strength in patients with femoroacetabular impingement and other hip pathologies

Article focus

• A review of the literature on methods of muscle strength measurement

• A closer focus on measuring muscle strength in patients with femoroacetabular impingement

• To identify key variables in measuring muscle strength, with the aim of standardising future research into the subject

Key messages

• Hip girdle muscles are weaker in patients with femoroacetabular impingement; this is the case in both hips, regardless of whether the hip is symptomatic or otherwise.

• Motor driven and hand held dynamometry are both reliable methods of muscle strength measurement, and should be used with make tests.

• Attempts should be made to standardise future research in muscle strength testing to ensure the ability to compare results across different studies. This includes employing the same tester for all measurements, stabilising the patient and identifying whether isometric or isokinetic results have been obtained.

Strengths and limitations

Strengths:

• A comprehensive review of all literature relating to muscle strength measurement

• An attempt to unify future research into measuring muscle strength

Limitation:

• The review is limited by the quantity and quality of the articles previously published. There were only two studies on the effect of FAI on hip muscle strength

Introduction

Femoroacetabular impingement (FAI) is a morphological osseous abnormality, which results in a physical conflict between the femoral head-neck junction and the acetabular rim.¹ The abnormality may be of the proximal femur (cam) or the acetabulum (pincer) or, most commonly, a mixture of both. This can result in, amongst other things, a chondrolabral lesion, which manifests as hip pain, and if not addressed can lead to osteoarthritis of the hip.^2,3 Hip arthroscopy has become a common procedure to treat the different aetiologies causing FAI.^4,5

The diagnosis of FAI is becoming more common and although the exact incidence is unclear, a study on asymptomatic individuals⁶ found an incidence of cam deformities of 24.7% in men and 5.4% in women. This is clearly a large proportion of the population who are at risk of developing the sequelae of the disease such as labral tears. More importantly, if FAI remains untreated, patients can suffer lifelong consequences in the form of osteoarthritis,^2,3 functional disabilities and risk of injury secondary to hip muscle weakness.⁷ Changes associated with an arthritic gait have been seen in patients affected with FAI.⁸

At presentation, patients with FAI are typically young adults who are healthy, active and classically partake in athletic activity.² They most commonly present with anterior groin pain but also experience limitations when performing activities of daily living and sport.^2,9

It has been shown that patients with FAI also present with hip muscle weakness,⁷ although currently data on this are limited. It is generally well known that patients with FAI have a reduced range of movement,¹⁰ however, to date only two studies have compared hip muscle strength in patients with FAI versus normal controls.^7,11 Both of these studies found reduced strength in patients with FAI compared with controls. It follows that, in accordance with current guidelines,^12,13 measuring hip muscle strength in patients with FAI could be of certain benefit.

A greater understanding of hip muscle strength could achieve a number of goals. Firstly, understanding the muscle strength changes associated with FAI will give a better understanding of the condition’s pathophysiology and will help to guide the development of future treatments. This is vital for a condition where 34% of patients have reported weakness that greatly limits activity.⁸ Secondly, at an individual level, measuring hip muscle strength could help guide and monitor patients’ individual treatment and its efficacy. Finally, if a specific pattern of muscle weakness associated with FAI is identified, measuring patient hip muscle strength could help with diagnosis. Philippon et al⁹ highlighted that the equivocal presentation of FAI leads to a risk of incorrect diagnosis which in turn can result in inappropriate tests and interventions.

There are currently multiple methods of measuring muscle strength described in the literature, with many variables. These variables include:

The strength measuring device:

• Manual muscle testing (MMT) – a subjective ‘grade’ of strength is given by an assessor for a given direction of joint movement, using a 5 point grading scale;¹⁴

• Hand-held dynamometry (HHD) – a calibrated pressure sensor is used by an assessor to measure the strength of a particular joint movement;

• Motor-driven dynamometry (MDD) – a calibrated pressure sensor is used to measure strength of movement; the sensor is held in position by a mechanical device (Fig. 1).

• The individual rater measuring strength;

• The types of movements performed;

  • Isokinetic – muscle contraction causing joint movement at constant speed;

  • Isometric – muscle contraction against resistance resulting in no movement;

• Body positioning and stability.

Fig. 1

Cybex dynamometer (a MDD) showing a participant during testing of hip flexors

The different variables in the methodology of measurement of muscle strength have different advantages and disadvantages, making them more or less suitable in specific situations and difficult to be used uniformly. The aim of this study, therefore, was to review the current literature available on this subject and to suggest guidelines on measuring muscle strength in patients with FAI, which would allow standardisation of the results available on this subject.

Materials and Methods

The Cochrane and PubMed databases were searched using the terms ‘hip’, ‘muscle’, ‘strength’ and ‘measurement’ in the ‘All Terms’ field and this produced 229 results. A further search using the terms ‘femoroacetabular’ or ‘impingement’ yielded 17 additional studies. It was decided to focus the review on recent literature and therefore the search was narrowed to papers published from 2000 onwards. This yielded 213 results. The abstracts were then reviewed by two medical researchers (EM and AM) independently to determine suitability for inclusion in the review. Articles were excluded if the abstract did not include any measurement of muscle strength. A total of 29 articles were deemed relevant to the study. The PRISMA chart is summarised in Figure 2.

Fig. 2

Preferred Reporting Items for Systematic Reviews and Meta-Analyses flow chart

All papers were individually assessed for strengths and weaknesses, both in terms of reliability and applicability to measuring muscle strength. Of particular interest were the methodology of hip muscle strength measurement used and the reliability of the results produced, measured by most papers as the intraclass correlation coefficients (ICC). This is a measure of correlation applied to quantitative data arranged into groups and is the standard method of assessing reliability of strength testing in the literature. The key conclusions drawn from the papers were then compared in order to produce the final conclusions of this review.

Results

A total of 29 articles were assessed, of which only three related to strength in FAI. A total of 14 further studies assessed hip muscle strength (six in hip pathology other than FAI, three in healthy adults and five in a paediatric population). A total of 12 studies assessed the reliability of the muscle testing protocol (the equipment or other aspects of the protocol). Some of these studies are summarised in Tables I and II^15-24 (the remaining studies can be found in the supplementary material).

Discussion

There is currently no standardised protocol described in the literature for measurement of muscle strength. A number of methods of measuring muscle strength have been described such as MMT, HHD, MDD and EMG.^{17-22,25,26,31-35,40} This causes difficulty in comparing the different studies as there are, at times, major differences in research methodology.

The most suitable method to measure hip muscle strength in patients with FAI will depend on the priorities of the situation and the aim of the measurement. The following recommendations from our evaluation should help to guide the choice of options available:

• MDD and HHD are both reliable methods of muscle strength measurement. However, the reliability of HHD depends on the consistency of the assessor and there is a high inter-observer error. In contrast, MDD eliminates this error and is therefore more reliable. However, HHD is more practical in the clinical setting. Both methods can be used reliably as long as these variations are accounted for.

• Isometric results are more reliable but isokinetic results are more representative of muscle action in vivo.

• Make tests should be used when feasible rather than break tests.

• It is important to standardise the measurement protocol, e.g. training of testers and verbal encouragement given to patients.

  • The same tester should be used for all measurements if possible.

  • The participants should be stabilised while measuring muscle strength.

• Specific muscle groups:

  • Hip flexion should be measured in the standing or supine position.

  • Hip extension is best measured in the standing or prone standing positions; the prone position is not recommended.

  • Internal and external rotation are more reliably measured in hip flexion, e.g. sitting.

Future work therefore needs to focus on developing current methods and also isolating variables to examine their effect on the reliability of results. Ideally, this would allow the direct comparison of specific variables, for example HHD versus MDD, while keeping other variables as similar as possible. A consensus needs to be agreed among the research community to ensure that comparable conclusions can be drawn from future research.

As highlighted by the literature, some muscle group measurements are still less reliable than others.^32,34 It is thought that this is due to poor stability and compensatory movements at the pelvis. Therefore, further work again needs to be done, focusing on these muscle groups, to improve the methodology with the aim of improving reliability. Another note on further work concerns the size of studies. The majority of studies involved a small number of participants, ranging from only two raters to 35 participants in comparative studies. Hébert et al³⁴ correctly identified that for these to be clinically useful, larger samples must be used in the future. In addition, Poulsen et al²⁵ suggested that to improve external validity, more than two clinicians should be used in future studies regarding inter-rater reliability. Finally, there are only two studies which have described muscle strength measurement and comparison in the context of FAI.^7,11 These comparative studies included 22 and 35 participants in each arm, however, despite the relatively low numbers, the results have shown significantly weaker hip muscles in FAI compared with normal controls. In addition, Harris-Heyes et al¹¹ identified weaker muscles in the unaffected hip of patients with FAI. This is surprising as patients with FAI are commonly athletic, and it merits further investigation. The paucity of research in this arena creates an exciting opportunity for future research. It must be highlighted that devising a standardised protocol for measuring hip muscle strength will improve the ability to compare future studies and enable a better understanding of the topic.

In conclusion, the most suitable method to measure hip muscle strength in patients with FAI will depend on the priorities of the situation and the aim of the measurement. There are a large number of variables in any situation; to be able to compare findings of different studies, it is pertinent to agree a set of ‘benchmarks’ in the measurement of hip muscle strength. Ideally, this should be in the form of a consensus between the centres where this research is undertaken.

From this review we would suggest that the following parameters are used in future research on the subject:

• MDD and/or HHD could be used to record the measurements but the assessor should remain constant if HHD is to be used;

• Make tests should be used and both isokinetic and isometric strength values should be recorded;

• Hip flexion and extension should be measured in standing (not prone);

• Hip rotation should be measured with hip in flexion (sitting position).